Generally, an air mat is an instrument that provides an appropriate cushion force through air pressure created by air injected into the inside thereof, so that a user can take a rest thereon comfortably. As such an air mat is convenient to carry and easily installed in every place, they are widely used.
Especially, recently, an air mat that-is widely used is provided with a plurality of embossing air pockets formed on a portion in contact with the human body so as to provide a finger-pressure effect to the user lying on the air mat and reduce unpleasant oppression against him or her.
As shown in FIG. 1, such a conventional air mat includes a cushion part 1 having a plurality of air pockets 1a arranged thereon, and a lower sheet 2 adapted to be bonded to the lower surface of the cushion part 1, for hermetically seal the lower portion of the cushion part 1.
Further, at one side of the cushion part 1 or the lower sheet 2 are provided air injection ports 3 for injecting air to the inside of the plurality of air pockets 1a. 
Moreover, each of the plurality of air pockets 1a communicates with the adjacent air pocket 1a by means of a communicating passageway 4, so that the air injected through the air injection ports 3 is supplied to the plurality of air pockets 1a through the communicating passageways 4.
Such a conventional air mat has generally been made by means of slush molding in such a manner that plastic resin (hereinafter, referred to as ‘plastic sol’) is first filled into a mold for forming the cushion part and is then preheated therein to a predetermined temperature. After that, when the plastic resin is partially coated to a predetermined thickness on the inner surface of the mold, the filled plastic resin is externally poured out of the mold, and the resin layer coated to the predetermined thickness is completely gelled and cooled, thereby primarily finishing the cushion part of the air mat.
Next, a plurality of communicating passageways are formed for allowing the plurality of air pockets formed in the cushion part to communicate with one another, and the lower sheet that has been previously made is then bonded to the lower surface of the cushion part, thereby finishing the whole structure of the air mat.
By the way, at the time of molding the cushion part, the conventional manufacturing method of the air mat using the slush molding is carried out in a state where the mold is opened at the upper portion thereof according to the characteristics of the slush molding, such that only after the cushion part is completely molded, the lower sheet can be bonded to the cushion part.
As a result, after the bonding process of the lower sheet and the cushion part are separately carried out, the air mat can be finally made.
More specifically, the bonding process of the lower sheet and the cushion part is conducted by means of high-frequency bonding or adhesive bonding. First, in case of the high-frequency bonding, a high-frequency bonding machine should be separately provided, and also, a relatively long time of period for the bonding process is necessary, such that the whole manufacturing processes are delayed, thereby raising the product costs undesirably.
Specifically, in case of the adhesive bonding, a relatively long time of period for the bonding process is necessary in the same manner as the high-frequency bonding, and since the bonding is mostly made by manual operations, it is difficult to provide the firm bonding of the lower sheet and the cushion part, which results in a low quality of bonding results.
If the plastic sol is removed from the mold according to the characteristics of the slush molding, lots of scraps remain irregularly on the surfaces of the respective air pockets into the mold.
Therefore, the air pockets do not have uniform thicknesses by the formation of the scraps thereon, so that they may be partially broken by the application of external pressures while in use.
Also, the plurality of communicating passageways should be formed for allowing the adjacent air pockets of the cushion part to communicate with each other, such that air can be gently supplied to each of the plurality of air pockets.
However, when the slush molding is adopted to mold the cushion part of the air mat, the shape of the cushion part after the molding is very simple, so that the communicating passageways should be made through a separate communicating passageway-forming process after the cushion part has been completely molded. Thus, the whole manufacturing processes are undesirably delayed and the product costs are accordingly raised.
Of course, a through-hole may be formed by cutting the partition between the adjacent air pockets into the mold, thereby also forming the plurality of communicating passageways.
In the same manner as mentioned above, however, lots of scraps are formed in the inner walls of the communicating passageways in the molding process thereof, which causes the communicating passageways to be irregular in thickness, thereby making them easily broken.
In order to solve these problems that the manufacturing method of the air mat by means of slush molding has had, thus, there has been proposed Korean Patent 25 Application No. 2005-0093038 entitled ‘an air mat and method for manufacturing and device of molding the same’, as filed by the same applicant as in this invention.
The prior art air mat manufacturing method is conducted not by means of slush molding but by means of rotational molding, wherein plastic sol is first poured into a mold and after a lower sheet and a cover are coupled to each other, the mold is rotated in a predetermined direction, so that the plastic sol is uniformly distributed in the mold by the centrifugal force during the rotation, thereby forming the cushion part, and at the same time, it is moved to the lower sheet, thereby smoothly bonding the lower sheet and the cushion part.
Therefore, the manufacturing method of the air mat using the rotational molding can overcome the problem that the cushion part and the lower sheet are separately bonded to each other in the slush molding, and at the same time, the bonding is carried out by the same material, thereby advantageously increasing the bonding force between the cushion part and the lower sheet.
However, this applicant has found that the following problems occurring when only the rotational molding is adopted.
First, the cushion part should be desirably formed thinly in thickness so as to be easily expanded and contracted, but according to the characteristics of the rotational molding, the manufacturing method of the air mat by means of the rotational molding fails to make the thickness of the cushion part relatively thin.
Further, it is difficult to make an article having a relatively small volume according to the characteristics of the rotational molding, thereby making it hard to mold the plurality of air pockets having substantially small volumes.
Additionally, separate communicating members should be formed in the mold so as to form the communicating passageways during the molding process, which causes unnecessary consumption of a material and undesirably increases the number of manufacturing processes.